Thermoassociative graft copolymers: NMR investigation and comparison with rheological behaviour

The thermoassociative properties of semidilute solutions of graft copolymers containing poly(N-isopropylacrylamide) (PNIPA) side chains are studied by H-1 NMR. Upon heating, the PNIPA grafts self-aggregate into hydrophobic microdomains and their H-1 NMR signal is no longer detectable. This phenomenon is attributed to the glassy structure of the aggregates' core and allows a direct detection of the aggregation process. On this basis it becomes possible to detect the formation of the microdomains by salting-out (with potassium carbonate) or their dissociation by adding an anionic surfactant (sodium dodecyl sulfate). The H-1 NMR observations are compared to the rheological data previously obtained and to small-angle neutron scattering measurements performed in similar conditions. A good correlation is evidenced between the macroscopic and microscopic transitions probed by the different techniques.